SUNDAY, April 26 (HealthDay News) -- U.S. health officials declared a public health emergency Sunday in response to the swine flu outbreak, as the number of confirmed cases nationwide rose to 20.

Homeland Security Secretary Janet Napolitano said the declaration was a precautionary measure, and did not mean that the threat posed by the outbreak was worsening. But, the move allows federal and state governments easier access to flu tests and medications, she said.

"That [a public health emergency] sounds more severe than it really is. This is standard operating procedure, and allows us to free up federal, state and local agencies and their resources for prevention and mitigation. It allows us to use medication and diagnostic tests that we might not otherwise be able to use, and it releases funds for the acquisition of additional antivirals," Napolitano said during a press conference at the White House.

Napolitano said the federal government had 50 million doses of the antiviral flu medication Tamiflu, and a quarter of those doses were being released to states, if needed, "particularly prioritizing the states where we already have confirmed incidence of the flu."

All 20 U.S. patients -- eight in New York, seven in California, two in Texas, two in Kansas and one in Ohio -- have recovered, Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, said at the news conference.

In a separate Sunday afternoon press conference, Dr. Anne Schuchat, the CDC's Interim Deputy Director for Science and Public Health Program, said that it's still too early to say that the flu outbreak in the U.S. will be less severe than the one occurring in Mexico.

"Right now it's premature to say the disease in Mexico is different than here. We don't have that many infected people at this point and we don't have great information from Mexico yet," Schuchat said. "Certainly, we have deaths in Mexico and we have not -- fortunately -- seen them yet here, but we fear that we may. We need to prepare for the idea that we will have additional cases, additional affected states and I do fear that we will have deaths here."

Meanwhile, in Mexico, believed to be the source of the outbreak, authorities continued to take dramatic steps over the weekend -- including suspending public gatherings -- to try to contain the swine flu outbreak that officials say has killed as many as 86 people, and sickened more than 1,400 others in that country.

In the United States, eight more cases of swine flu, all involving school students, were confirmed Sunday by New York City health officials, while another case was reported in Ohio. Two cases were reported in Kansas on Saturday, plus another in California. That brings the national total of confirmed swine flu cases to 20. All of the cases have so far been mild.

Some of the U.S. cases involved people who had recently returned from trips to Mexico, Schuchat noted in the Sunday press conference. The two cases reported in Kansas involved a husband and wife who had recently been to that country, she said. And The New York Times reported that some of the students at St. Francis Preparatory School, in Queens, had recently come back from Mexico as well.

Also on Sunday, Canadian officials confirmed four "very mild" cases of swine flu at a school in Nova Scotia, and two other cases in British Columbia. According to the Associated Press, a provincial health official said that the infection that sickened the students in Nova Scotia "was acquired in Mexico, brought home and spread."

The CDC's Besser said that as the number of cases of swine flu continues to grow in Mexico, his agency expects to see more cases in the United States. "As we look for cases of swine flu, we are seeing more cases of swine flu, and we would expect to see more cases of swine flu," he said.

"We have ramped up our surveillance around the country to try and understand better what is the scope, what is the magnitude of this outbreak," Besser said.

Although all the reported cases in the United States have so far been mild, there are bound to be more severe cases, Besser said. "Given the reports out of Mexico, I would expect that over time we are going to see more severe disease in this country," he said.

Napolitano said the Department of Homeland Security has started "passive surveillance protocols to screen people coming into the country."

"All persons entering the United States from a location of human infection of swine flu will be processed by appropriate CDC protocols," she said. "Right now these are passive. They are looking for people and asking about: 'Are you sick? Have you been sick?' and the like. And if so, they can be referred over for further examination. Travelers who do present with symptoms will be isolated."

Despite the outbreak the U.S. government has not told people not to travel to Mexico or other counties where flu has been found. "To date the State Department had not issued official travel advisories, for particularly Mexico, but these situations are very fluid," Napolitano said.

There are steps people can take to help prevent catching and spreading the flu, including frequent hand-washing, Besser said. "If you are sick it is very important that people stay at home. If your children are sick, have a fever and flu-like illness, they shouldn't go to school. And if you are ill you shouldn't get on an airplane or another public transport. Those things are part of personal responsibility in trying to reduce the impact," he said.

In a Saturday news conference, Schuchat told reporters that, because of the wide geographic spread of the virus so far, the outbreak was already "beyond containment."

But she added that U.S. health officials had numerous tools to fight the illness' spread and protect the health of Americans. The viruses found in the United States are resistant to two antiviral medications -- amantadine and rimantadine -- but are susceptible to the antivirals oseltamivir (Tamiflu) and zanamivir (Relenza), Schuchat said.

Schuchat said steps were already being taken to devise a vaccine against this strain of swine flu, although the process takes time. "We are taking the initial steps in terms of preparing the seed virus to hand off to the industry partners, to produce large quantities. But you know it takes months to produce a vaccine," she said.

In response to the developments in North America, countries around the world planned quarantines, tightened rules on pork imports and tested airline passengers for fevers as global health officials tried Sunday to come up with uniform ways to battle the outbreak. Nations from New Zealand to France reported new suspected cases and some warned citizens against travel to North America, the AP said.

World Health Organization Director-General Margaret Chan held teleconferences with staff and flu experts around the world but stopped short of recommending specific measures to halt the disease beyond urging governments to step up their surveillance of suspicious outbreaks, the news service said.

In Mexico, the government has ordered schools closed and all public events have been suspended for the time being, including more than 500 concerts and other gatherings in the city of 20 million residents. Even churches stood empty Sunday, the AP said.

While Mexico's flu season is usually over by now, health officials noticed a sizeable uptick in flu cases in recent weeks. According to a report published in The New York Times on Friday, World Health Organization experts said that most deaths among Mexican patients with swine flu have involved healthy young adults.

That could be worrisome, experts say. Seasonal flus usually strike hardest at infants and the elderly, but pandemic flus -- such as the 1918-19 Spanish flu, which killed an estimated 20 million to 40 million people worldwide -- often strike young, healthy people, the newspaper reported.

Swine flu is a respiratory disease of pigs caused by type A influenza. Swine flu does not normally infect humans. However, human infections do occur, usually after exposure to pigs. Symptoms resemble those of the regular flu, including sore throat, coughing and fever.

SOURCES: April 26, 2009, White House press conference with Homeland Security Secretary Janet Napolitano, and Richard Besser, M.D., acting director of the U.S. Centers for Disease Control and Prevention; April 26 and 25, 2009, teleconferences with Anne Schuchat, M.D., director, U.S. Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases, Atlanta; The New York Times; Associated Press